The present invention relates to a semiconductor integrated circuit and, more particularly, to a technology for preventing the malfunction of a semiconductor integrated circuit resulting from a change in the electrical characteristics thereof.
In designing a semiconductor integrated circuit, a so-called signal integrity (SI) verification is performed which verifies and guarantees that a malfunction due to noise does not occur by performing simulation (see Japanese Laid-Open Patent Publication No. 9-305649). On the other hand, a noise margin improvement method based on the control of a substrate voltage has been proposed. In this case, to implement a constant threshold voltage or saturation current irrespective of a temperature condition and process conditions, the substrate voltage is determined by performing feedback control using a reference circuit (see Japanese Laid-Open Patent Publication No. 2001-156261).
There has also been proposed a method which uses a flip-flop for detecting the occurrence of a set-up error on a specified path and performs feedback control to increase a power supply voltage when a set-up error is detected (see “A Self-Tuning DVS Processor Using Delay-Error Detection and Correction” by Das, S. et al., 2005 Symposium on VLSI Circuits Digest of Technical Papers, IEEE, Jun. 16, 2005, pp. 258-261).
When feedback control is thus performed, it is assumed that each of a reference value and a feedback function is uniquely determined. For example, for the control of a substrate voltage, a reference voltage is generated by using a band gap reference circuit utilizing the fact that the band gap of a semiconductor is fixed as a physical constant. In relation to the feedback function, it has been performed that, e.g., the relationship between the substrate voltage and a threshold voltage or between the substrate voltage and a saturation current is implemented with an analog circuit or the relationship between a power supply voltage and a set-up error is implemented with critical paths and a set-up error detection flip-flop.
However, in the case of guaranteeing that no malfunction occurs by simulation, it is necessary to guarantee that “a malfunction does not occur” in such a manner as, for example, “there is totally no problem” or “there is statistically no problem when a time period of a given length is considered”. This leads to the addition of an excessive margin and thereby causes the degradation of the performance of a circuit.
A problem associated with signal integrity is caused not only by transistor characteristics but also by mutual influencing characteristics such as a wire-to-wire capacitance and the relative relationship between driving abilities. This causes the problem that it is difficult to produce a reference value for a capacitance, a reference value for the relative relationship between driving abilities, or the like or it is difficult to perform adaptive control in response to a use situation because the feedback function cannot be uniquely determined. Additionally, in the case of performing correction in consideration of signal integrity, it is necessary to perform correction not only to the transistor characteristics but also to variations in wire-to-wire capacitance. However, although the control of the substrate voltage allows the correction of the transistor characteristics, it is impossible to correct the variations in wire-to-wire capacitance only with simple substrate voltage control.
Moreover, it has become increasingly difficult to predict the state of a circuit as the miniaturization of the circuit proceeds so that the influence of problems as mentioned above has become greater.